Television receiver regulations in the United States call for future television receivers to be able to process signals which contain information in encoded data format within line 21 of the scanned television raster, referred to as "line 21 information," from which there may be generated a display of closed captioning information (Federal Communications Commission Report and Order on GEN Docket No. 91-1, dated Apr. 12, 1991). The data contained in raster line 21 may be video related, in which case it is referred to as "captions", or non-video related, in which case it is referred to as "text".
In order for a television receiver to make use of the line 21 information, it is necessary to locate the signal portion representing raster line 21 containing the data, to extract the data, to decode it, and to transform it into the appropriate alphanumeric characters, which are then incorporated into the demodulated video signal for display on the screen. The proper recovery, identification, and placement of the characters on the screen requires accurate timing and synchronization with the horizontal and vertical timing of the composite video signal and therefore makes necessary a stable timing reference and a highly accurate extraction of timing information from the incoming composite video signal.
Television video waveforms, called "composite video," contain horizontal, vertical, and field synchronization information, along with the picture information. FIG. 1 is an example of such a composite video signal. The synchronization information portion of the waveform, which includes both horizontal and vertical synchronization is referred to as "composite synchronization." A schematic example of a horizontal interval waveform 11, labeled as such, is illustrated in greater detail in FIG. 1a.
A serious difficulty in the extraction of composite synchronization from a composite video signal is that such signals as are available in a television receiver frequently contain considerable extraneous noise, particularly impulse noise. In addition, the signal amplitudes may vary widely. A receiver system which is able to respond properly to such varying amplitudes is referred to as being "adaptive."
The composite synchronization can be extracted from the composite video by means of a voltage clamping circuit, or "clamp", and a voltage comparator. The most negative part of the video waveform, referred to as the "synchronization tip", or "sync tip" is caused by appropriate electronic circuits to be set, or "clamped" to a reference voltage, so that all of the synchronization tips, or at least the average voltage of the synchronization tips in the case of a noisy signal, are set to this clamping voltage level shown in FIG. 1b in relation to other voltage features of the signal. A second reference is chosen to be at a higher voltage than the clamping voltage and preferably at a level exactly halfway between the clamping level and the level of the back porch voltage. This second voltage level is called the "slice level." If the video waveform is applied to the positive input of a voltage comparator and the slice level voltage is applied to the negative input of the voltage comparator, as shown in FIG. 1c, then whenever the video waveform voltage is more positive than the slice level, a positive voltage appears at the output of the comparator. Conversely, when the video waveform voltage is more negative than the slice level, a negative voltage appears at the output of the comparator. The resulting output of the comparator is a "squared-up" version of the lower portion of the video waveform and is what is considered the composite synchronization, as shown in FIG. 1d.
While the timing signal pulses normally are easily distinguished by the comparator from the remainder of the composite video signal, this process is vulnerable to signals with noise added. Additional difficulties with existing video signal timing extraction circuits which are addressed by the present invention will also be discussed below.
The extraction of the data which has been encoded into the television raster line 21 requires several operations. Typically, along with the data itself, there is also included information about the bit, or clock rate and the byte boundaries. The recovery process requires that the internal clock be synchronized to the transmitted bit rate, that the timing of the byte boundaries be established, and that the proper slicing level be set to enable the recovery of data even under adverse conditions of signal level deterioration and noise. The synchronization of the internal clock timing to the transmitted bit rate clock timing constitutes a timing recovery process referred to as "data clock recovery". The two clocks are usually at the same frequency, but they must be brought into a mutual phase lock condition, so that the extraction circuits can sample the sliced data at the optimum time to achieve the optimum data recovery in a noisy environment.
The video signal input to the data recovery circuits is usually already clamped by the previous circuit action. The establishing of the slice level for use in the voltage comparator for extracting the data is more difficult than it is in the synchronization recovery process, however, because the data occurs infrequently, once per television frame, and for only short periods of time during line 21. Good data recovery depends on an adaptive data slice level which can adjust quickly to variations in data amplitude in a given line 21 while also being able to hold the slice level between successive frames. Although circuits have been devised which perform these functions, the additional requirement that such a circuit perform well within an integrated CMOS environment adds serious complications. There is a need for a sample-and-hold circuit which performs all the needed functions without the need for components external to the circuit chip which are normally required.
The internal timing signals needed to perform all of the processing for data recovery and display, also referred to as the video "dot clock," are derived from a single, stable, high frequency timing source reference. This stable timing reference is normally a VCO (voltage-controlled oscillator) whose frequency is established by a crystal or by other stable discrete components and which is then phase-locked to the composite video synchronization signal. This timing reference is used in the data extraction phase and also in generating jitter-free characters for display. The requirements for achieving such a stable VCO are easily met in a discrete environment, but are usually difficult to achieve in an integrated circuit whose designing involved the primary objectives of minimum size and minimum number of lead connections. There is therefore a need for a circuit which provides the required VCO performance within a CMOS environment.